Method for adjusting lithographic mask flatness using thermally induced pellicle stress

ABSTRACT

A method for adjusting the flatness of a lithographic mask includes determining an initial mask flatness of the mask, determining an applied stress for bringing the mask to a desired mask flatness, and determining a mounting temperature of a pellicle frame to be mounted to the mask, the mounting temperature corresponding to the applied stress. The actual temperature of the pellicle frame is adjusted to the determined mounting temperature, and the pellicle frame is mounted to the mask at the mounting temperature.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional application of U.S. application Ser.No. 10/905,453, filed Jan. 5, 2005, now U.S. Pat. No. 7,355,680, thecontents of which are incorporated by reference herein in theirentirety.

BACKGROUND

The present invention relates generally to semiconductor deviceprocessing techniques and, more particularly, to a method for adjustinglithographic mask flatness using thermally-induced pellicle stress.

Semiconductor fabrication techniques often utilize a mask or reticle ina conventional lithographic system to project an image onto asemiconductor wafer, wherein radiation is provided through (or reflectedoff) the mask or reticle, and passed through a focusing optical systemto form the image (e.g., an integrated circuit pattern). Thesemiconductor wafer is positioned to receive the radiation transmittedthrough (or reflected off) the mask such that the image formed on thewafer corresponds to the pattern on the mask. The radiation source maybe light, such as ultraviolet light, vacuum ultraviolet (VUV) light,extreme ultraviolet light (EUV) and deep ultraviolet light (DUV). Inaddition, the radiation may also be x-ray radiation, e-beam radiation,etc. Generally, the formed image is utilized on the wafer to pattern alayer of material, such as a photoresist material. The photoresistmaterial, in turn, may be utilized to define doping regions, depositionregions, etching regions, or other structures associated with themanufacture of integrated circuits (ICs).

Reticle flatness has become increasingly important as lithographic focuswindows shrink. A smaller process window is undesirable forsemiconductor manufacturing where process drifts could shift theoperating point away from the optimal dose and/or focus range. Thesmaller the process window, the more likely yield loss will occur whenthe process drifts. Mask non-flatness consumes some of the processwindow; for example, a typical specification for mask blank flatness isless than 2 microns. At this maximum allowable value, the resultingimpact is about 175 nanometers (nm) at the wafer. However, this value ison the order of the entire focus budget for some critical mask levels.On the other hand, a reticle flatness of 0.5 microns or bettercorresponds to less than 30 nm impact at the wafer, which is moretolerable.

It is undesirable to rely solely on incoming substrate flatness to meetdesired tolerances. Flatter masks are more expensive to order, andbecause they push technology limits, they are not always within thetight specification limits. Moreover, both mask processing and pelliclemounting processes contribute to adverse changes in flatness. A pellicleis a thin, optically-transparent membrane used to protect patternedphotomask surfaces from contamination by airborne particles. Typically,the pellicle includes a metal (e.g., aluminum) frame having one or moreof the walls thereof securely attached to a chrome side of the mask orreticle. The membrane is stretched across the metal frame and preventsthe contaminants from reaching the mask or reticle. Since the particlesthat fall on the pellicle are out of focus, they do not distort theimage printed on the wafer.

Unfortunately, the mounting of the pellicle frame can alter maskflatness by exerting mechanical stresses on the mask. For example,recent technical articles have described how pellicles can affect theshape of the mask as a function of initial pellicle flatness andtemperature change (Cotte et al., Experimental and Numerical studies ofthe Effects of Materials and Attachment Conditions on Pellicle-InducedDistortions in Advanced Photomasks, SPIE Vol. 4754, pp. 579-588 (2002)).

Accordingly, it would be desirable to implement a pellicle mountingprocess wherein reticle flatness is not adversely affected and, evenmore advantageously, wherein existing reticle flatness may be improvedfollowing pellicle attachment.

SUMMARY

The foregoing discussed drawbacks and deficiencies of the prior art areovercome or alleviated by a method for adjusting the flatness of alithographic mask. In an exemplary embodiment, the method includesdetermining an initial mask flatness of the mask, determining an appliedstress for bringing the mask to a desired mask flatness, and determininga mounting temperature of a pellicle frame to be mounted to the mask,the mounting temperature corresponding to the desired applied stress.The actual temperature of the pellicle frame is adjusted to thedetermined mounting temperature, and the pellicle frame is mounted tothe mask at the mounting temperature.

In another embodiment, an apparatus for adjusting the flatness of alithographic mask includes a pellicle frame configured for mounting tothe mask. The pellicle frame has a plurality of sides including amaterial having a selected coefficient of thermal expansion so as toinduce a determined stress on the mask. The determined stress causes aninitially measured mask flatness to be adjusted to a desired maskflatness.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring to the exemplary drawings wherein like elements are numberedalike in the several Figures:

FIG. 1 is a block diagram illustrating a method for adjustinglithographic mask flatness using thermally induced pellicle stress, inaccordance with an embodiment of the invention;

FIG. 2 is a schematic block diagram illustrating inputs and outputs of amathematical model used in the method of FIG. 1;

FIGS. 3( a) through 3(c) depict an exemplary application of the methodof FIG. 1;

FIG. 4 is a graph illustrating mask flatness before and after adjustmentby thermally-induced pellicle stress; and

FIGS. 5( a) through 5(f ) illustrate various pellicle frameconfigurations and materials, in accordance with a further embodiment ofthe invention.

DETAILED DESCRIPTION

Disclosed herein is a method for improving photolithographic maskflatness through the use of thermally-induced pellicle stresses.Although the effects of pellicle attachment have heretofore beenrecognized as having a negative impact on mask flatness, the presentinvention embodiments affirmatively utilize the differences intemperature between the pellicle frame during mask mounting and theambient temperature of the photolithographic mask to improve maskflatness.

Referring initially to FIG. 1, there is shown a block diagramillustrating a method 100 for adjusting lithographic mask flatness usingthermally induced pellicle stress, in accordance with an embodiment ofthe invention. As shown in block 102, the mask flatness of an individualmask is mapped, prior to mounting a pellicle frame thereon. In thismanner, individual variations in flatness from mask to mask may bespecifically compensated for. Then, as shown in block 104, amathematical model is applied in order to determine an optimal stress(magnitude and direction) to be applied by the pellicle frame to themask, in order to bring the mask to a desired degree of flatness.

Inputs to the mathematical model 200 (depicted schematically in FIG. 2)include stepper temperature, the mask flatness data determined in block102 of FIG. 1, and the physical properties of the pellicle and maskmaterials. The physical properties used by the model include the elasticmodulus, Poisson's ratio, coefficient of thermal expansion (CTE) andphysical dimensions of the mask, pellicle frame and adhesive materialused to attach the pellicle frame to the mask. The model 200 uses theseinputs to determine a particular pellicle frame stress needed to resultin a desired mask flatness. Depending on whether an expansive force or acontractive force is desired, the pellicle frame temperature will bechanged with respect to stepper temperature. Thus, another output ofmodel 200 is a calculated pellicle frame temperature that will result inthe pellicle frame stress when the pellicle is mounted to the mask.

It should be noted at this point that the adhesive material is selectedso as to efficiently transfer stress from the pellicle frame to themask, and will thus have a relatively high elastic modulus (e.g., inexcess of 1 MPa). One suitable example for such an adhesive would be anacrylic polymer.

Returning to FIG. 1, method 100 proceeds to block 106, wherein thepellicle (frame) mounting temperature for producing the optimal stressfor desired mask flatness is determined. At block 108, the pellicleframe temperature is then set using a heating and/or cooling source. Asdescribed in further detail hereinafter, the application of stressingforces applied to the pellicle frame has multiple degrees of freedom,depending on the application of heating/cooling sources and/or physicalcharacteristics of the frame itself. Then, once the desired pellicleframe temperature is verified, the pellicle is mounted to the mask atblock 110.

Because the coefficient of thermal expansion (CTE) for an aluminumpellicle frame is about 24 ppm/° C. while that for the quartz mask isabout 0.6 ppm/° C., it will be appreciated that expansion or contractionof the pellicle frame may be used to intentionally alter the flatness ofthe mask. A schematic depiction of this effect is illustrated in FIGS.3( a)-3(c). In FIG. 3( a), a cross-sectional view of a mask 302 is shownhaving non-ideal flatness such that the flatness measurements thereofreveal a convex shape. Upon application of the above described model, acertain pellicle frame mounting temperature is determined for the givenmask such that the pellicle-mask system will result in a flatter maskafter cooling to ambient temperature. In this instance, since the maskis measured to be convex, a “hot” pellicle temperature will be used,since the pellicle frame will contract upon cooling to the steppertemperature. Thus, as shown in FIG. 3( b), the heated pellicle frame 304is mounted to the convex mask 302. Once the frame 304 begins to cool,the pulling contraction causes the mask 302 to become flatter, as shownin FIG. 3( c).

Conversely, it will be appreciated that for a concave mask, an attached“cold” pellicle frame will expand as it warms to the steppertemperature, thereby pushing the mask and correcting the concavitythereof. Because the CTE for Al is roughly 40 times that of quartz, thepellicle and mask could be processed at the same temperature, therebysimplifying implementation.

The exemplary data in the table of the first example presented belowestablishes the feasibility of such a procedure. In lieu of a quartzmask material, the initial flatness of a pair of 100 mm silicon waferswas mapped at room temperature (due to the availability of waferflatness measuring equipment). Then, a pellicle frame was mounted toeach wafer, one at room temperature and the other at an elevatedtemperature of 45° C. After cooling the high temperature pellicle frame,the flatness of the two wafers was once again mapped at roomtemperature:

EXAMPLE 1

Initial wafer flatness Final wafer flatness Change in range (max-mm)Temperature of range (max-mm) wafer without pellicle pellicle duringwithout pellicle flatness (μm) mounting (° C.) (μm) (μm) 6.9 20 7.3 0.47.4 45 21.2 13.8

As summarized in the table above, the pellicle frame mounted at roomtemperature had little effect on the initial wafer flatness (only about0.4 microns), while the pellicle frame mounted at 45° C., and followedby cooling, altered the flatness of the wafer by almost twice itsoriginal value.

Once the desired pellicle frame temperature is determined, it could beachieved by controlling the temperature of the entire pellicle mountingtool and environment or, alternatively, by heating or cooling thepellicle frame directly. One suitable example of a heating method wouldbe through electrical resistance (i.e., by passing an electric currentthrough the frame). An example of a cooling method would be throughthermal conduction by contacting the frame with a cold substance such ascold air or a cold solid. In implementing a temperature-changingmechanism, the actual pellicle frame temperature may be measured bycontacting the frame with a measuring device such as a thermocouple orby viewing it with an infrared (IR) device. The ambient temperature nearthe frame could also be measured. Once the determined frame temperaturehas been attained, the pellicle frame is mounted to the mask. Thetemperature measurement could be fed back to the heating or coolingsystem to create a feedback loop to drive the system temperature to aset point.

The second example presented below establishes the feasibility of a2-dimensional mathematical model (as described in FIG. 2) applied to amask. In particular, mask deflection simulations were run using thestandard, continuum model for thermal expansion of solid films ofdiffering mechanical properties in two spatial dimensions (incross-section for the embodiment herein). The constitutive equations forstress and deflection are discretized for numerical solution via thefinite element method (as may be found in TSUPREM-4 User's Manual,Version 6.5, May 1997, pp. 2-95, Technology Modeling Associates, Inc.Sunnyvale, Calif., the contents of which are incorporated herein intheir entirety. Values of mechanical properties for each film materialwere input from the literature, including the Cotte reference describedabove (e.g., Young's modulus, poisson ratio and thermal expansioncoefficient).

The initial non-flatness of the mask is depicted in the graph 400 ofFIG. 4 by curve 402. A pellicle was mounted to the mask at a temperaturethat is about 20° C. below the temperature of the stepper. Once thepellicle/mask assembly warmed up to the stepper temperature, thepellicle frame expanded, thereby exerting a stress on the mask thatimproved the mask's flatness. Prior to the mounting of the pellicle, thedeviation of the central 90 mm of the mask was about 1.2 μm, as shown bycurve 402. However, after the pellicle was mounted to the mask andsubsequently warmed up to the stepper temperature, the deviation of thecentral 90 mm of the mask improved to about 0.14 μm, as shown by curve404 of FIG. 4. The material property inputs used by the mathematicalmodel in determining applied pellicle frame stress and pellicle frametemperature are as follows:

EXAMPLE 2

Mask Material: quartz

Mask Dimensions: 152 mm×152 mm×6.3 mm

Mask Elastic modulus: 72.6 Gpa

Mask CTE: 0.55 ppm/° C.

Pellicle Frame (aluminum)

Pellicle Frame Dimensions: length=149 mm, span=122 mm, thickness=6.1 mm,width=2 mm

Pellicle Frame Elastic modulus: 72 Gpa

Pellicle Frame CTE: 23.6 ppm/° C.

It will be noted that in the above example, the mathematical modelassumes that the adhesive is 100% effective in transferring stress fromthe pellicle frame to the mask.

Finally, in accordance with a further embodiment of the invention, FIGS.5( a)-5(e) illustrate various pellicle frame configurations andmaterials that could be implemented, depending upon the specific initialflatness of the mask. Depending upon the outcome of the modelingprocess, it may be that the resulting pellicle frame temperature neededto bring the mask to the desired flatness is of such a value as torender the process impractical. As such, the physical characteristics ofthe pellicle frame itself may be adjusted in order change the stressversus temperature effects of the frame.

For example, if a greater stress is desired at a lower temperature, apellicle frame 502 (FIG. 5( a)) having a standard frame thickness may bereplaced with another pellicle frame 404 having an increased thickness,as shown in FIG. 5( b). Moreover, FIG. 5( c) illustrates a pellicleframe in which sides 506 along a first direction have a differentthickness than sides 508 along a second direction. Each of the foursides could also have different thickness to further customize theamount and direction of the force applied to the frame by the mask. Thisresults in an additional degree of freedom with respect to optimizingflatness in orthogonal directions.

Still another alternative is to utilize a pellicle frame havingmaterials with different coefficients of thermal expansion, such asshown in FIGS. 5( d) and 5(e). Whereas the sides 510 in one directioncould be made of a material such as aluminum, the sides 512 disposed inthe other direction may be made of another material such as quartz, forexample, such that the stress applied in the x and y directions would bedifferent. As is the case for tailored thickness, each of the fourindividual sides could also be made from different materials.

FIG. 5( f) illustrates a further pellicle frame embodiment, wherein ahigh CTE, electrically conductive material (e.g., aluminum) is used foreach of the sides 514 of the frame, while an electrically and thermallyinsulating material is used at the corners 516 of the frame. Certaindesirable properties of the corner material include: electrically andthermally insulating, compatibility with the pellicle adhesive, andnon-particle generating. Thus configured, the temperature of each of thefour sides 514 of the frame may be controlled independently, byindividualized electrical resistance heating, for example. This againresults in more degrees of freedom with respect to optimizing flatness.

It will further be appreciated that although the invention embodimentsdescribed herein are presented with respect to a pellicle frameattachment for mask flattening, it is contemplated that the principlesare equally applicable to future generation masks and structures whichdo not specifically utilize a pellicle frame in the fabrication thereof.

While the invention has been described with reference to a preferredembodiment or embodiments, it will be understood by those skilled in theart that various changes may be made and equivalents may be substitutedfor elements thereof without departing from the scope of the invention.In addition, many modifications may be made to adapt a particularsituation or material to the teachings of the invention withoutdeparting from the essential scope thereof. Therefore, it is intendedthat the invention not be limited to the particular embodiment disclosedas the best mode contemplated for carrying out this invention, but thatthe invention will include all embodiments falling within the scope ofthe appended claims.

1. An apparatus for adjusting the flatness of a lithographic mask,comprising: a pellicle frame configured for mounting to the mask; saidpellicle frame having a plurality of sides, at least a portion of whichcomprising a material having a different coefficient of thermalexpansion with respect to the mask so as to provide one of an expansiveforce or a contractive force to the mask; wherein said one of anexpansive force or a contractive force provided by said pellicle framecauses an initially measured mask flatness to be adjusted to a desiredmask flatness.
 2. The apparatus of claim 1, wherein said sides of saidpellicle frame disposed in a first direction have a first thickness andsaid sides of said pellicle frame disposed in a second direction have asecond thickness.
 3. The apparatus of claim 1, wherein said sides ofsaid pellicle frame disposed in a first direction comprise firstmaterial and said sides of said pellicle frame disposed in a seconddirection comprise a second material having a different coefficient ofthermal expansion than said first material.
 4. The apparatus of claim 1,wherein said first material is aluminum and said second material isquartz.
 5. The apparatus of claim 1, wherein said sides of said pellicleframe comprise an electrically conductive material and corners of saidpellicle frame comprise a thermally insulating material.
 6. Theapparatus of claim 5, wherein said sides of said pellicle frame areconfigured to be heated by passing an electrical current therethrough.7. The apparatus of claim 6, wherein said sides of said pellicle frameare electrically insulated from one another.